The present invention relates to the art of color image processing. It finds particular application in conjunction with digital highlight color printers and copiers, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Mapping full-color images to black and white allows rendering and/or reproduction of the full-color image without the associated costs of color processing. That is to say, generally, monochromatic, or black and white, rendering and/or reproduction are faster and more economical than full color. Additionally, full-color processing may not always be available. Likewise, it is advantageous to be able to map full-color images to a highlight-color scheme. Highlight-color schemes, such as those produced by the Xerox 4850 Highlight Color Printer, generally include only two color separations as opposed to the three, four, or more separations commonly associated with full-color image processing. One such technique for mapping full-color images to highlight-color images is disclosed in commonly assigned U.S. Pat. No. 5,237,517, incorporated herein by reference.
As indicated earlier, images rendered in a highlight-color scheme are typically portrayed by two color separations, a highlight color (usually a primary color) and a base color (usually black or white). On the other hand, in a full-color image, the gamut of colors includes tints and shades of the full color spectrum--reds, greens, blues, and their combinations. The gamut of full colors is commonly represented by a three-dimensional volume or color space which takes the form of the double hexagonal cone 10 illustrated in FIG. 1. In this representation, shade or luminance varies from dark to light along the vertical axis, tint varies from unsaturated grays to fully saturated colors along the radial axis, and hue varies angularly in the horizontal plane. The gamut of colors available in a highlight-color scheme is represented by the two-dimensional triangle 12 illustrated in FIG. 2. This two-dimensional triangle 12 represents a slice from the full-color double hexagonal cone 10 at the angle of the hue being used for the highlight color.
The mapping from full color to highlight color is analogous to mapping from the three-dimensional double hexagonal cone 10 to the two-dimensional triangle 12. In such a mapping, many different colors in the full-color space are mapped to the same color in the highlight-color space, and information is lost. It is advantageous then to have a mapping that preserves the information that is most important, thus minimizing damage due to the loss of information that occurs when mapping from a three-dimensional color space to a two-dimensional color space. The information that should be preserved depends on the type of image and how color is being used. For pictorial images, such as photographs and the like, most of the information is in the luminance. Of secondary importance is the hue. However, in presentation images, such as graphs, charts, and the like, the information is often in the hue. In this application, usually strongly saturated colors are selected for impact, and hue is used to differentiate elements of the image. Colors that differ in luminance as well as hue are often selected to increase their distinguishability. When color is used to distinguish an object, the information lies in the fact that the object is colored and not neutral. In any event, different aspects of the color space are important in different applications.
Prior art techniques for mapping full-color images into a highlight-color space generally use complex computations and/or large three-dimensional look-up tables. This being the case, the prior art techniques have a number of inherent drawbacks. One such drawback is that the complex computations are relatively time consuming and often become a bottleneck which slows the rendering process. Additionally, large three-dimensional look-up tables are relatively burdensome and costly. Measures taken to reduce the size of the three-dimensional look-up table also tend to reduce quality and/or performance. Moreover, due to its size, large amounts of memory are allocated to store the three-dimensional look-up table.
The present invention contemplates a new and improved mapping technique, which overcomes the above-referenced problems and others.